Menopause, the transition in women from the reproductive to the non-reproductive stage of life, is characterized by the cessation of menstruation and occurs at an average age of fifty years. The postmenopausal state is characterized by changes in the levels of circulating sex hormones, the most dramatic of which is the reduction in plasma levels of 17.beta.-estradiol to less than ten percent of premenopausal values. Clinical and epidemiological studies have shown that the postmenopausal state is an important risk factor for a number of chronic disorders, notably osteoporosis and cardiovascular disease. In view of the fact that the current life span of women is about eighty years, women spend approximately one-third of their lives in the postmenopausal state. This means that the potential for chronic effects of the postmenopausal state on women's health is greater today than at the turn of the century when life expectancy was considerably shorter.
Osteoporosis describes a group of diseases which are characterized by the net loss of bone mass per unit volume. The consequence of this loss of bone mass and resulting bone fracture is the failure of the skeleton to provide adequate structural support for the body. The most vulnerable bone tissue to the effects of postmenopausal osteoporosis is the trabecular bone. This tissue is often referred to as spongy or cancellous bone and is particularly concentrated near the ends of the bone (near the joints) and in the vertebrae of the spine. The trabecular tissue is characterized by small osteoid structures which inter-connect with each other, as well as the more solid and dense cortical tissue which makes up the outer surface and central shaft of the bone. This inter-connected network of trabeculae gives lateral support to the outer cortical structure and is critical to the biomechanical strength of the overall structure.
Following the cessation of menses, most women lose from about 20% to about 60% of the bone mass in the trabecular compartment of the bone within 3 to 6 years. This rapid loss is generally associated with an increase of bone resorption and formation. However, the resorptive cycle is more dominant and the result is a net loss of bone mass.
In postmenopausal osteoporosis, it is primarily the net resorption and loss of the trabeculae which leads to the failure and fracture of bone. In light of the loss of the trabeculae in postmenopausal women, it is not surprising that the most common fractures are those associated with bones which are highly dependent on trabecular support, for example the vertebrae, the neck, and the weight bearing bones such as the femur and the fore-arm. Indeed, hip fracture, collies fractures, and vertebral crush fractures are hallmarks of postmenopausal osteoporosis.
There are an estimated 25 million women in the United States alone who are afflicted with this disease. The results of osteoporosis are personally harmful and also account for a large economic loss due its chronicity and the need for extensive and long term support (hospitalization and nursing home care). This is especially true in more elderly patients. Additionally, although osteoporosis is not generally thought of as a life threatening condition, a 20% to 30% mortality rate is related with hip fractures in elderly women. A large percentage of this mortality rate can be directly associated with postmenopausal osteoporosis.
Cardiovascular disease is the leading cause of death among women. Compared to men, premenopausal women are relatively protected from cardiovascular disease; however, this protection is gradually lost following menopause. This loss of protection has been linked to the loss of estrogen and, in particular, to the loss of estrogen's ability to regulate the levels of serum lipids. The nature of estrogen's ability to regulate serum lipids is not well understood, but evidence indicates that estrogen can up-regulate the low density lipid (LDL) receptors in the liver which act to remove excess cholesterol. Additionally, estrogen appears to have some effect on the biosynthesis of cholesterol, and other beneficial effects on cardiovascular health. It has been reported in the literature that serum lipid levels in postmenopausal women having estrogen replacement therapy return to concentrations found in the premenopausal state.
At the present time, the most generally accepted method for treatment of disorders resulting in the postmenopausal state from the decline in estrogen levels is estrogen replacement therapy. The therapy may take the form of administering estrogen alone in so-called unopposed estrogen replacement therapy (ERT) or in the form of coadministering estrogen and progestin in a so-called hormonal replacement therapy (HRT) regimen. There are, however, major liabilities associated with chronic administration of estrogen in postmenopausal women having to do with adverse effects on reproductive tissues, namely breast and uterus. Women on ERT develop endometrial cancer at rates three to six times higher than nonusers after three to six years of use; after ten years of ERT, the risk ratio increases to tenfold. A growing body of literature suggests that long-term ERT (10-15 years) causes a thirty to fifty percent increase in the risk of breast cancer.
To combat these deleterious effect of ERT, the coadministration of progestin along with estrogen in a combined hormonal replacement therapy (HRT) is employed, since progestin acts to limit uterine stimulation and thus reduce the risk of uterine cancer.
Because of these known and suspected or feared liabilities of estrogen therapy, prescription of and patient compliance with chronic estrogen replacement therapy has been poor. It has been estimated that, in the United States among postmenopausal women for whom ERT or HRT has been prescribed, fewer than forty percent continue therapy beyond one year.
As a consequence, there is a need for the development of postmenopausal therapy agents which possess the ideal pharmacological profile: for example agents which produce the beneficial effects of estrogen upon vasomotor systems, skeletal tissue and the cardiovascular system without producing the adverse effects of estrogen upon reproductive tissues. Agents possessing such an estrogen profile would reverse the effects of estrogen deficiency in certain tissues while at the same time bypassing or failing to act in tissues in which estrogen produces adverse effects. The term selective estrogen receptor modulators or "SERMs" has been applied such compounds which possess this tissue selective profile. SERMs are defined as compounds producing estrogen agonism in one or more desired target tissues such as bone, liver, etc., together with estrogen antagonism and/or minimal (i. e. clinically insignificant) agonism in reproductive tissues such as the breast or uterus.